MIL-DTL-38999: Connecting the Past to the Future of Military Aviation

In 2004, as the aerospace industry celebrated 100 years of flight, Michael A. Clarke chronicled the evolution of military aviation for the National Academy of Engineering. He wrote that the advancements achieved in aviation during the 20th century were greater perhaps than all the breakthroughs in medicine, chemistry, physics and other scientific fields.

His premise was that technological advancement, dictated by the operational needs of military aviation, was critical to the United States achieving and maintaining an aerial advantage over its rivals. He wrote, “In the case of the United States we have reached a fundamental decision point on the vector for future military aviation, and moving forward will require all of the engineering skills we have developed over the last century.”

Although Clarke does not mention it specifically, it can be argued that no single invention has played a larger role in enabling the evolution of military aerospace technology than one of its smallest components: the circular connector.

From the earliest days of flight, avionics consisted largely of a series of instruments that relayed conditions of the aircraft to the pilot. In less than 10 years from the Wright brothers’ days in Kitty Hawk, airplanes were being used to fight wars over Europe. Immediately, engineers were searching for ways to give combat pilots improved performance and maneuverability.

Soon airspeed, altitude, temperature and pressure gauges and other electromechanical instruments filled the cockpits, with each one dedicated to a single purpose. By the 1930s, the means for connecting the boxes had been standardized by the Departments of War and Navy. For their compact size, durability and reliability, four series of circular connectors were adopted. Quickly, the MIL-C-5015 connector became the industry standard for providing power to the various boxes and was instrumental in improving the performance and safety of combat aircraft.

The thinking of avionics as separate boxes prevailed through World War II as the role of military aircraft was generally restricted to aerial combat, transportation or the dropping of conventional bombs. As the role of the aircraft changed, however so did the need for improved avionics.

Although the jet engine was invented in the 1930s, the introduction of the jet fighter after the Korean War changed everything. Aircraft were not just thought of as a means to extend the battlefield vertically; they became a platform for fighting a new kind of war. The Cold War saw the rapid development of radar, missile technology and “smart” bombs that could destroy targets on the ground with greater precision and fewer munitions. Bombers were retrofitted to carry nuclear payloads, and jet aircraft were at the heart of the perfection of close air support. The role of military aviation evolved into the delivery platform for many of these new capabilities, and the days of independent boxes vanished.

The connectors evolved along with the aircraft to enable networked boxes. The MIL-C-26482 specification was next in the 1950s (Navy driven spec), and MIL-C-83723 came soon after in the 1960 ead by the Air Force to differentiate from the Navy, with both being very similar connectors.

Rather than simply providing power as they had previously, new connectors would be required to accommodate smaller wires while carrying signals at faster speeds. The 5015 connector was great for power, but when signal and higher speeds were needed, it was limited due to size 16 contact being the smallest offered. The boxes would process information and communicate with each other to create a feedback loop, so MIL-C 26482 and MIL-C 83273 qualified connectors were developed to introduce size 20 contacts, which carried less power but had the ability transmit signal. By the 1960s, pilots found themselves in airframes wrapped around computers.

More contact arrangements were configured. Density increased. The contacts themselves changed to allow data to flow between boxes faster and more reliably. As boxes proliferated to handle the explosion of information necessary to keep planes aloft and alive. By the 1970s, more signal and less power in the input/output connectors was needed, and the MIL-DTL-38999 standard was born. It offered size 22 contacts that allowed for higher speed data transmission along smaller gauge wire. The 38999 also brought the ability to handle higher vibration profiles via more robust coupling mechanisms and mating threads, so planes could fly faster and higher, knowing the interconnects would survive.

For 38999 connectors, the biggest challenge remained signal carrying capability within a compact packet with small gauge wires and contacts, but this was just the beginning. Connectors adapted to meet higher vibration requirements, improved EMI shielding effectiveness with RFI bands and increased durability, fluid resistance and environmental sealing. Engineers improved the shell and shell bottom for best mating and limiting motion between mated connectors to increase resistance to vibration for longer lasting connectors. The electromechanical approach to avionics was officially over and the “fly by wire” era began with the introduction of the F-16 fighter.

The next great turning point for 38999 connectors came in the 1980s and 1990s as economic and environmental pressures began to impact the evolution of military aircraft. Seeking fuel efficiency to allow for longer flight time, as well as the desire to carry greater payloads, engineers would need to develop lighter materials for everything from the airframe down to the connectors themselves. Advancements in materials science would usher in composite materials to replace the stainless steel and aluminum construction of the 38999’s shells.

Anti-pollution measures would also impact the 38999 circular connector in the 1990s. Long admired for their durability and corrosion-resistance based on cadmium coatings, the 38999s were adapted to a variety of nickel-zinc coatings to comply with policies that required the reduction of cadmium usage.

The pressures to adapt the connectors exist today as the military’s use of aircraft and the weaponizing of information become more inextricably linked. Unmanned aerial vehicles networking with ground systems, C4ISR applications and even wearable soldier systems are forcing 38999 connectors to be continually redesigned. For example, 38999 connectors meet the demand of fast Gigabit Ethernet speeds of 10Gb/s, as required by many modern system protocols.

Figure 2: Deutsch series iii connectors

One of the many interesting breakthroughs comes recently from TE Connectivity (TE) and its next generation ACT composite connector. The ACT series circular connectors (part of the well known deutsch series of 38999 series III connectors) are one of the most versatile circular mil spec connectors in the industry. Their composite shells provide up to 40% weight savings over aluminum shells, and provide excellent corrosion resistance and high durability, offering 1,500 mating cycles. TE’s next generation mold tooling helps to reduce the variation seen in other molded materials, which allows the ACT connectors to offer a more consistent performance. The metallization of the composite material through the plating process has been advanced as well and offers one of the most consistent connectors to hit the market.

There is no doubt that as the battlefield environment gets more complex, connectors will continue to play a vital role in the future of American air superiority. Furthermore, as the civilian uses of the 38999 connector—from commercial aerospace to autosports to marine exploration—multiply, and the evolution of sensors and the volume of data that systems process increases, engineers will continue to reexamine how this consistent performer can be used to power (and signal) the next great evolution.

Scott Miller is product manager of global Aerospace, Defense and Marine at TE Connectivity. Connect with him at scott.miller@te.com or visit TE’s military connectors page atwww.te.com